1. Field of the Invention
The present invention relates to a work pallet changer for machine tools such as a vertical or horizontal machining center, a milling machine, a planer, a shaper, a boring machine or a drilling machine, and is adopted to change the work pallet on the operation table of such a machine tool.
2. Description of the Prior Art
In recent years, machine tools have had to improve their operational efficiency as their power, speed and cost prices are increased.
Therefore, this invention is intended to reduce the down time of a machine tool during the work changing process using work pallets on which one or more workpieces are fixed. That is, one or more workpieces are fixed on or removed from a work pallet outside the machine tool while the operation of the machine tool continues. Then, after the machining operation is finished, the workpieces and the associated pallets will be changed by means of the work pallet changer.
The work pallet changer for machine tools is basically comprised as follows:
A pallet fixing device is provided on the work table of a machine tool so as to support and position a work pallet. A pallet shifting system, located in front of the machine tool, comprises a plurality of pallet mounting parts, each of which is moved between a pallet-changing position in front of the pallet-positioning device and a retracted position remote from the pallet-changing position. Each pallet is moved between the pallet mounting part located at pallet-changing position and the pallet positioning device.
In this basic arrangement, there are several types of known mechanisms for moving each pallet-mounting part between the pallet-changing position and the retracted position, including: I. A horizontally rotating type of the type disclosed in U.S. Pat. No. 4,090,287, as shown herein in FIG. 31 (Prior Art I). As depicted in that figure, two pallet mounting locations 101 are disposed on the front side and rear side, respectively, of a rotary table 102. The rotary table 102 can be rotated about its central vertical axis 103 in a horizontal plane by a half revolution. Besides the pallet-positioning device 106 is supported for rotation in a horizontal plane around a vertical axis 109 above the table 105 of the machine tool 104. Initially, a work pallet 107 holding finished workpieces is moved from the pallet-positioning device 106 to a pallet-mounting location 101 that is positioned in a pallet-changing station X on the rotary table 102. The rotary table 102 is then turned a half a revolution, so that the work pallet 107 holding the finished workpieces is moved to the retracted position Y and another work pallet 108 holding unfinished workpieces is moved to the pallet-changing station X. Next, the changing of work-pallets 107 and 108 on the table 105 is completed by moving the work pallet 108 holding the unfinished workpieces from the pallet-changing station X to the pallet-positioning device 106. II. An elevation type as shown in FIG. 32 (Prior Art II) wherein two pallet-mounting elements 111 properly separated from each other in a vertical direction are disposed on the top wall part and the bottom wall part of an elevating frame 112. III. A horizontally rotating conveyance type as shown in FIG. 33 (Prior Art III) wherein six pallet mounting elements 121 are disposed along a conveyer line 123 of a horizontal rotating conveyer 122 in spaced relation.
However, the prior art discussed above has various disadvantages. Specifically, in the horizontally rotating type shown as Prior Art I and the horizontally rotating conveyance type shown as Prior Art III, a plurality of pallet-mounting parts are disposed in side-by-side relation in a horizontal plane, and an excessively large space is required for their revolution or circulation, so that a larger area is required to fit the pallet-changer in the space in front of the machine tool.
In case of elevation type shown as Prior Art II, the workpiece changing operation at the lower pallet-mounting part is inefficient because both the right and the left side walls of the elevation frame 112 interfere with the operation. In order to improve the efficiency of this pallet changing operation, the distance between these side walls must be expanded laterally beyond the width of the work pallet requiring the elevation frame to be made larger and heavier, and the drive system for elevating the heavy frame must be larger and more powerful, with more power consumption.
In either of the horizontally rotating types of prior art, the elevation type or the horizontal circular conveyance type, removal of chips is difficult and can make work conditions bad, because the chips must be swept out, washed out or blown out from above the upper surface of the work pallet.
Furthermore, in order to improve the efficiency of chip-removing operation, it is sometimes required that a chip removing system also be installed at different stages, to insure removal of chips from the work pallet. In the latter case, a chip-removing system and an extensive conveyor are a necessary addition that is expensive, and requires excessive space for installation. Moreover, the time and labor required by a double-trip conveyance of the work pallet between the pallet changer and the chip-removing system is excessively difficult.